1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus and a process cartridge.
Here, an electrophotographic image forming apparatus is the one that forms an image on a recording medium (for example, a recording paper, an OHP sheet, a cloth and the like) employing an electrophotographic image forming method. Examples of the electrophotographic image forming apparatus include an electrophotographic copying machine, an electrophotographic printer (for example, a laser beam printer, an LED printer and the like), a facsimile machine and a compound machine thereof (multi-function printer).
In addition, a process cartridge is a cartridge into which the electrophotographic photosensitive member and at least one of a charging device, a developing device, and a cleaning device functioning as a process unit acting on this electrophotographic photosensitive member are integrally incorporated. Further, this cartridge is to be detachably mountable to an electrophotographic image forming apparatus main body.
2. Description of the Related Art
In an electrophotographic image forming apparatus, from the advantage of reproducibility of a half-tone image or suppression of an excessive edge effect of an image, a contact development method is widely employed. The contact development method means the system in which a developer carrying member included in a developing device is brought into contact with an electrophotographic photosensitive member, and a latent image having been formed on the electrophotographic photosensitive member is developed using a developer (toner).
The contact development method, in order for an electrophotographic photosensitive member and a developer carrying member to be evenly in close contact in the rotational axis direction of the electrophotographic photosensitive member and the developer carrying member, is generally arranged such that one member is an elastic member (inclusive of e.g., a sheet that is backed up by an elastic member) and the other is a rigid member. The simplest construction is such a construction that the electrophotographic photosensitive member is a rigid member and the developer carrying member is an elastic member. In particular, in the case where a process cartridge that is detachably mountable to an image forming apparatus main body is constructed to incorporate an electrophotographic photosensitive member and a developer carrying member, the above-mentioned combination is frequently employed in the process cartridge.
In a cylindrical developer carrying member (developing roller) to be attached to an opening portion of a developing frame the process cartridge includes, there are roughly two ways of developer seal methods at the end portion in the rotational axis direction (hereinafter, it is referred to as a longitudinal direction) of the developer carrying member.
One is a developing roller circumferential surface seal method. In this method, used is a developing roller longer than the longitudinal length of the above-mentioned opening. Further, in a developing frame, both end portions that are provided to be opposed to the circumferential surface of the developing roller are formed to be circular-arc shaped, and a seal member such as a felt material or a moltopren is bonded to each of these end portions. Moreover, this seal member is pressed to the circumferential surface of the developing roller to seal a developer.
The other one is a developing roller cross section seal method. In this method, used is a developing roller rather shorter than the longitudinal length of the above-mentioned opening. Further, in the developing frame, with both side walls that are provided to be opposed to the cross section (cross section orthogonal to the longitudinal direction) of the circumferential surface of the developing roller where a developer is carried, the shaft core that is provided inside of the circumferential surface portion of this developing roller is rotatably held. Moreover, there is located a seal member such as a felt material or a moltopren in a gap between both side walls and the developing roller circumferential surface portion to seal a developer (toner).
Although the above-mentioned two seal methods have merits or demerits of a seal performance of the developer, the developing roller circumferential surface seal method is excellent in terms of the ease of assembly. In the case of employing the developing roller circumferential surface seal method, there are produced on the circumferential surface of the developing roller the area where the developer is carried (developer coat area) and the area where no developer is carried (developer non coat area).
In addition, there is a possibility that a slight developer is stuck onto the electrophotographic photosensitive member to be contacted with the developing roller as a matter of course. Therefore, the longitudinal length of a cleaning blade (cleaning member) serving as a cleaning device to be contacted with the electrophotographic photosensitive member to remove the developer on the electrophotographic photosensitive member, not depending on the above-mentioned two seal methods, is usually set to be larger than the longitudinal length of the developing roller (for example, Japanese Patent Application Laid-Open No. H06-230650). Exceptionally, as disclosed in Japanese Patent Application Laid-Open No. H06-230650, there is an example in which the length of the developing roller is shorter than the length of the cleaning blade. Japanese Patent Application Laid-Open No. 2005-221858 is what solves an inherent problem that is generated at the time of using an electrophotographic photosensitive member having flexibility.